1. Field of the Invention
The present invention relates generally to semiconductor circuits and, more particularly, to reliable interconnect via structures and methods for making the same.
2. Description of the Related Art
Interconnect structures of integrated circuits (ICs) generally take the form of patterned metallization lines that are used to electrically interconnect devices and to provide interconnection with external circuitry. By way of example, IC devices may include metal oxide semiconductor ("MOS") devices having diffused source and drain regions separated by channel regions, and gates located over the channel regions. In practice, an IC chip may include thousands or millions of devices, such as MOS transistors.
Conventionally, a dielectric layer (e.g., silicon oxide) is deposited over the devices, and via holes are patterned and formed through the dielectric layer to the devices below. As is well known in the art, photolithography "patterning" is typically accomplished by depositing a photoresist layer over the dielectric layer, selectively exposing the photoresist to light through a patterned reticle having via hole patterns, developing the photoresist to form a resist via mask, and etching the exposed dielectric layer to form via holes leading to a lower level. Once the via holes are formed, a conductive material such as tungsten is used to fill the via holes to define what are known as "W" tungsten plugs. Once the, tungsten plugs are formed, a metallization layer is formed over the dielectric layer and the tungsten plugs. The metallization layer is then patterned using conventional photolithography techniques to define a first level of interconnect metal routing. This process may then be repeated if additional layers of metallization lines are desired.
Recently, to reduce via resistance and increase device speeds, designers have been filling the via holes with aluminum "Al", and using low "K" dielectric materials for the intermetal dielectric layers. The aluminum filled via holes (i.e., aluminum plugs) and the low "K" dielectric materials have been successful in decreasing resistance in interconnect structures, but a substantial amount of aluminum atoms within the aluminum plugs have been found to migrate along with the flow of electrons. This electron flow therefore causes the formation of voids in the interconnect lines as well as in the aluminum plugs.
To illustrate this problem, FIG. 1 shows a cross-section of a semiconductor substrate 10 having a plurality of conventionally fabricated layers. The semiconductor substrate 10 may include diffusion regions 12 and a polysilicon gate 14 formed over the semiconductor substrate 10. A first dielectric layer 19 is formed over the semiconductor substrate 10 and is then planarized. Once planarized, via holes are formed through the first dielectric layer 19, and an aluminum plug 16 is defined after a chemical vapor deposition (CVD) aluminum deposition. Next, a metallization layer is deposited and patterned over the first dielectric layer to define a first level of interconnect lines 24. The process is again repeated to form a second dielectric layer 22, aluminum plugs 18a and 18b, and a second level of interconnect lines 26 Once the structure is complete, current "I" may be passed through the interconnect structure formed by the first and second levels of interconnect lines 24 and 26, and the aluminum plugs 18a and 18b. Therefore, when current flows that are typical in interconnect buses, power lines "Vdd" and ground lines "Vss" are passed through this low resistive structure, the electron flow "e" may unfortunately cause voids 32 in the aluminum plug 18a and voids 30 and 30' in the interconnect lines 24 and 26, respectively. It is believed that the voids 32 in the aluminum plug 18a is partially due to the fact that less aluminum material is contained within the via holes as compared to the interconnect line itself. By way of example, a typical aluminum plug may contain a volume of about 0.75 microns.sup.3 of aluminum, while a typical metallization interconnect line lying over an aluminum plug may contain a volume of about 3.2 microns.sup.3.
Accordingly, when the current flow begins to cause electromigration of aluminum atoms in the interconnect structure of FIG. 1, an "open circuit" failure will necessarily tend to occur much more rapidly within the aluminum plug 18a. In practice, given the above exemplary aluminum plug and interconnect volumes, aluminum plugs may cause device failures within about 1 to 2 years of use, while the electromigration in the interconnect line may not cause a device failure for more than about 10 years. However, a device is only as reliable as its weakest link, and therefore, even though the interconnect lines can withstand more severe voids due to electromigration, the aluminum plugs will unfortunately cause the entire device to fail at a much faster rate, thereby producing a less reliable device.
Another interconnect problem that has recently caused designers substantial difficulties is accidental over-etching of aluminum plugs when a misalignment occurs in the photolithography process. As shown in FIG. 1, when the metallization layer that is patterned to form the second interconnect layer 26 is misaligned, the underlying aluminum plug 18a is left exposed to the etching steps that are used to pattern the second interconnect layer 26. Because the aluminum plug 18a is essentially the same type of aluminum-based material used for the second interconnect layers 26, the etching chemistries used will also attack the aluminum plug 18a. When this happens, a gap 40 is may be etched into the aluminum plug 18a, which may enable process chemicals and moisture to be trapped therein. Of course, when chemicals or moisture are trapped within gap 40, aluminum plug failures may arise due to corrosion and vapor energy releasing explosions during subsequent high temperature operations. For more information on vapor energy releasing explosions, reference may be made to a commonly assigned U.S. patent application Ser. No. 08/856,949, filed on May 15, 1997 and having inventors Subhas Bothra and Ling Q. Qian. This application is incorporated by reference herein.
Accordingly, in view of the foregoing, there is a need for a highly reliable aluminum plug technology that assists in decreasing interconnect resistance while preventing aluminum plug failing voids. Further yet, there is a need for an aluminum plug technology that prevents destructive gaps in the aluminum plugs due to photolithography misalignments.